The question of "How do I change the transmission fluid on my Puch E50?"comes up a lot. Here's an illustrated how-to which should explain the process.
First, read the manual
The manual linked here has illustrated directions for this process on page 7 which you should read. These directions probably negate the purpose of the rest of this article, but whatever.
Second, start your engines
You want to start the engine and get it warmed up for a bit. This will make the oil flow better during the draining process. If for some reason you can't start it now but you still want to change the fluid, that's not the end of the world. Just keep an eye on it and check to make sure the new stuff doesn't get too contaminated by anything that didn't drain out with the old stuff.
Once your engine is warmed up, turn it off.
Third, remove the plugs
There are two screws that act as plugs. One is in the drain hole, and one is in the fill hole. Remove the screw in the fill hole first, then the plug from the drain hole. Have a jar or something handy before you remove the drain hole plug, because the fluid is going to come pouring out that drain hole once you get the screw past a certain point.
Fourth, replace drain plug
Put the drain plug back in. You want it snug, but don't go crazy tightening it because you can strip out the hole.
Fifth, add oil
Puchs use Type F transmission fluid. This should be readily available at just about any auto parts store (and many other places as well).
For an E50, use 5.74 oz (170cc) of oil. Add this oil into the fill hole. If you are too lazy to measure, you can just fill it until it starts overflowing from the fill hole. When this happens, wait until it stops overflowing, and then you should have approximately the right amount of oil.
Replace the fill hole plug/screw, and you should be good to go.